linux-kernel - Re: [RFC v2 1/2] dma-buf: Introduce dma buffer sharing mechanism

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Message-ID: <CAF6AEGugC4hW-NUU4Zss=ACSCrqads+=nwULGRaMhhTX-1uP+g@mail.gmail.com>
Date:	Mon, 5 Dec 2011 16:35:03 -0600
From:	Rob Clark <rob@...com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-arm-kernel@...ts.infradead.org, t.stanislaws@...sung.com,
	linux@....linux.org.uk, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	linaro-mm-sig@...ts.linaro.org, linux-media@...r.kernel.org,
	Sumit Semwal <sumit.semwal@...aro.org>,
	m.szyprowski@...sung.com
Subject: Re: [RFC v2 1/2] dma-buf: Introduce dma buffer sharing mechanism

On Mon, Dec 5, 2011 at 4:09 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Monday 05 December 2011 14:46:47 Rob Clark wrote:
>> I sort of preferred having the DMABUF shim because that lets you pass
>> a buffer around userspace without the receiving code knowing about a
>> device specific API.  But the problem I eventually came around to: if
>> your GL stack (or some other userspace component) is batching up
>> commands before submission to kernel, the buffers you need to wait for
>> completion might not even be submitted yet.  So from kernel
>> perspective they are "ready" for cpu access.  Even though in fact they
>> are not in a consistent state from rendering perspective.  I don't
>> really know a sane way to deal with that.  Maybe the approach instead
>> should be a userspace level API (in libkms/libdrm?) to provide
>> abstraction for userspace access to buffers rather than dealing with
>> this at the kernel level.
>
> It would be nice if user space had no way to block out kernel drivers,
> otherwise we have to be very careful to ensure that each map() operation
> can be interrupted by a signal as the last resort to avoid deadlocks.

map_dma_buf should be documented to be allowed to return -EINTR..
otherwise, yeah, that would be problematic.

>        Arnd
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> dri-devel@...ts.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/dri-devel
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